1. Field of the Invention
This invention relates to a recording and/or reproducing apparatus using, for example, a disc-shaped record bearing medium, and, more particularly, to a head positioning method.
2. Description of the Prior Art
It is known in the art of a recording and/or reproducing apparatus using a disc-shaped record bearing medium, for example, a magnetic disc, to provide a device for shifting a magnetic head to permit concentric or helical recording of signals or reproducing the thus-recorded signals as constructed with a rotative-to-linear motion gearing transmission between a stepping motor and a carrier for the magnetic head so that the head can be shifted stepwise along the radius of the magnetic disc.
For the adjustment of position of the magnetic head to be controlled without detecting the absolute position, it is also known to provide an open loop control system in which the initial position of the magnetic head is adjusted to a particular setting with the stepping motor in the corresponding angular position, and the operation of an up/down counter for memorizing the position of the head is controlled in accordance with the driving of the stepping motor.
In this case, therefore, it is of great importance to locate the head in the start position with high accuracy. And, the conventional method of determining whether or not the head is set in the start position, is to employ photo-electric transducer means arranged to cooperate with the head carrier.
Meanwhile, as the recording density is increased by the recent development of magnetic sheets of higher quality and magnetic heads of improved performance, the track width and track pitch are narrowing. Under such a situation, the accuracy of adjustment of the initial position of the magnetic head is required to further increase, and an auto-tracking capability must be introduced to assure constant establishment of the on-tracking condition when in reproduction. As far as achievement of the latter is concerned, the amount the head moves each time the stepping motor rotates one step may be made reasonably smaller than the track pitch by assigning a plurality of drive steps of the motor to every one track pitch. With the use of such auto-tracking system, if the amount by which the head shifts when it is brought into setting in the start position is left unchanged from that equal to one track pitch, the following problem will, however, arise.
Taking, as an example, the video floppy disc having concentric fifty tracks of 60 .mu.m wide with a pitch of 100 .mu.m, in a recording and/or reproducing apparatus having a stepping motor whose one step represents an angle of rotation of 18.degree. and a speed reduction for shifting the magnetic head one track pitch by pitch for every 12 steps of the motor, and assuming that when in recording, a train of drive pulses with a frequency lying in a self-excitation range are applied to the stepping motor so that the magnetic head is shifted stepwise by a unit of one track pitch, and, when in reproduction, the position of the head is adjusted with an accuracy of the unit of one step, namely, 100/12 8.6 .mu.m in order to insure that the output of the magnetic reproducing head is optimum.
Further assuming that when bringing the magnetic head to the start position, the accuracy of detection of when the start position is reached, is left equal to the unit of one track pitch, then, if it happens that the phase of the rotor of the stepping motor relative to the stator thereof is caused to accidentally change by a shock or vibration of larger force than the residual torque of the stepping motor given to the apparatus when not in use, the use of the open loop control system for the stepping motor will result in the adjustment of the magnetic head to such an erroneous start position as is different from the true one by an amount corresponding to the changed value of phase, because movement of the magnetic head toward the start position begins at a position of deviated phase when the apparatus is switched on again. Since the accuracy with which the adjustment of the magnetic head to the start position is as rough as unity of track pitch, the magnetic head is caused to access the start position with an error of integer multiples of 8.6 .mu.m in amount from the true one, that is, with inclusion of a deviation.
And, when recording goes on from such an erroneous start position, the magnetic head does not coincide with any of the positions that all the tracks should take respectively.